Stabilized red phosphorus and process for making it

The present invention relates to stabilized red phosphorus and to a process for making it, wherein red phosphorus, which is in the form of an aqueous suspension, is treated with a metal salt of a phosphinic acid of the general formula ##STR2## in which R 1 and R 2 each stand for an alkyl, cycloalkyl, aryl or aralkyl group, as a stabilizing agent.

It has been described that red phosphorus in moist atmosphere undergoes a chemical surface reaction involving oxidation and disproportionation with the resultant formation of various acids of phosphorous (oxidation stages +1 to +5) and hydrogen phosphide.

As described by Gmelin, Handbuch der anorganischen Chemie, 8th edition (1964), vol. phosphorus, part B, page 83, Verlag Chemie, Weinheim (Bergstrasse), red phosphorus can be stabilized by means of aluminum hydroxide which is precipitated on the phosphorus particles by the consecutive addition of aqueous 10% sodium hydrogen carbonate and aluminum sulfate solutions heated to 55°-60° C. The resulting aqueous suspension is filtered and the filter residue is dried. This process is, however, not satisfactory in respect of the following points. In order to produce a satisfatory stabilizing effect, it is necessary to use relatively large quantities of aluminum hydroxide which are undesirable inasmuch as they contaminate the phosphorus to an unacceptable extent and adversely affect is further widespread uses.

A further process for stabilizing red phosphorus has been described in U.S. Patent Specification 2,359,243, wherein red phosphorus is suspended in an aqueous 0.04 normal solution of sodium aluminate. Next, a stream of air is passed for 10 hours at 85° to 90° C. through the suspension, which is filtered, washed with hot water and dried under vacuum.

A still further process for stabilizing red phosphorus has been disclosed in U.S. Pat. No. 2,635,953, wherein aluminum hydroxide is used in combination with zinc or magnesium hydroxide.

The processes last described do equally not permit red phosphorus to be satisfactorily stabilized against oxidation with the use of a minimum of stabilizer.

The known oxidation stabilizers have more specifically an insufficient thermal stability and liberate water at higher temperatures, which is disadvantageous. In those cases in which plastics material rendered flame-retardant by means of red phosphorus, which in turn has an oxidation stabilizer incorporated therein, are to be processed on an extruder, it is an imperative requirement that the oxidation stabilizer combine in itself thermal stability with undecomposability and non-liberation of water, even at temperatures higher than 300° C.

In accordance with our present invention, we have unexpectedly found that red phosphorus can satisfactorily be stabilized by precipitating a minor proportion of a metal salt of a phosphinic acid of the general formula ##STR3## in which R 1 and R 2 have the meanings given hereinabove.

The present invention relates more particularly to stabilized pulverulent red phosphorus consisting of a homogeneous blend of red phosphorus particles with a size of at most about 2 mm and a metal compound of the second or third group of the Periodic System as an oxidation stabilizer, wherein the metal compound is the aluminum, calcium or zinc salt of a phosphinic acid of the general formula ##STR4## in which R 1 and R 2 each stand for an alkyl, cycloalkyl, aryl or aralkyl group having 1 to 20 carbon atoms, preferably 1 to 12 carbon atoms, the metal compound being present in the homogeneous blend in a proportion of 0.5 to 5 weight %, preferably 0.5 to 3 weight %.

The radicals R 1 and R 2 may stand for identical or different, branched and/or unbranched groups which may optionally be substituted, the substituents comprising halogen, such as chlorine or bromine, or a carboxyalkyl-group.

The useful phosphinic acid salts comprise, e.g. the salts of bis-(chloromethyl)-, 2-carboxyethylmethyl- or octylmethyl phosphinic acid.

The red phosphorus is preferably used in the form of particles with a size of about 0.01 to 0.15 mm.

The invention also relates to a process for making stabilized pulverulent red phosphorus consisting of a homogeneous blend of red phosphorus particles with a size of at most 2 mm and a metal compound of the second or third group of the Periodic System as an oxidation stabilizer, which comprises: intimately blending red phosphorus particles with a particle size of at most about 2 mm with about 0.5 to 5 weight %, based on red phosphorus, of a phosphinic acid of the general formula ##STR5## in which R 1 and R 2 each stand for an alkyl, cycloalkyl, aryl or aralkyl group having 1 to 20 carbon atoms, preferably 1 to 12 carbon atoms, suspending the blend in water and heating the resulting suspension to about 60° to 95° C.; admixing the suspension with at least stoichiometric proportions of an aqueous solution of a water-soluble aluminum, calcium or zinc salt to cause precipitation of the respective salt of phosphinic acid from the aqueous suspension at a pH of 3.0 to 7.5; filtering the resulting mixture, and drying the filter residue at elevated temperature and, if desired, under reduced pressure.

The red phosphorus is more preferably used in the form of particles having a size of about 0.1 to 0.15 mm and its aqueous suspension is preferably heated to a temperature of 80° to 90° C. A further preferred feature of the present process provides for a proportion of 0.5 to 3 weight % of metal compound to be present in the mixture with red phosphorus.

To prepare the phosphinic acid metal salts, use may be made, for example, of Al 2 (SO 4 ) 3 . 18 H 2 O, Ca(NO 3 ) 2 . 4 H 2 O or ZnSO 4 . 7 H 2 O, the salts being used in the form of aqueous 5 to 20 weight % solutions. It is good practice to precipitate the phosphinic acid metal salts from the aqueous suspension within a certain pH-range as these metal phosphinates are partially soluble in an acid aqueous medium. The aluminum salts of phosphinic acid should more preferably be precipitated from the aqueous suspension at a pH-value of 3.0 to 3.5, and the calcium or zinc salts of phosphinic acid should be precipitated at a pH-value of 7 to 8 or 5 to 6.

A further preferred feature of the present process finally provides for the filter residue obtained after filtration of the aqueous suspension to be dried at a temperature of 80 to 130° C.

The stabilized red phosphorus and the process of the present invention for making such phosphorus compare favorably with the prior art products and methods inasmuch as the stabilizer is a thermally stable compound which is the first to permit the incorporation of stabilized red phosphorus into plastics at processing temperatures higher than 250° C.

The following Examples illustrate the invention.

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1 0.20 3.7

2 0.18 4.7

3 0.23 3.5

4 0.47 7.2

5 0.39 6.3

6 0.37 5.5

7 0.52 7.0

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The figures in column A of the above Table indicate the quantity of hydrogen phosphide (mg PH 3 /g . h) which is evolved on subjecting the phosphorus specimens to oxidation.

The figures in column B of the above Table relate to the acidity of the aqueous phosphorus-containing suspensions, which is caused by the formation of phosphoric acids on subjecting red phosphorus to oxidation (mg KOH/g . h).

As it would appear from the figures in columns A and B, the phosphinic acid compounds of the present invention are capable of stabilizing red phosphorus considerably.